An ecosystem is a community of living organisms interacting with the non-living elements of their physical environment. Life within these boundaries requires a constant supply of two fundamental components: energy and matter. Organisms need energy for work, such as growth and movement, and chemical elements to build their bodies. Understanding how energy flows and matter cycles is essential to grasping how natural systems function.
Energy Movement Through Ecosystems
The ultimate source of energy for almost all ecosystems is sunlight, which enters the system through primary producers like plants and algae. These organisms convert solar radiation into chemical energy—sugars—through photosynthesis. This chemical energy is then stored in the producer’s biomass, making it available to the food web.
Energy moves sequentially through trophic levels, beginning with producers and moving to consumers. When an organism consumes another, stored chemical energy is transferred. This transfer is inefficient because a significant portion of the energy is lost at each step.
Approximately 90% of the energy consumed is used by the organism for metabolism, respiration, and movement, and is transformed into heat. Only about 10% of the energy from one trophic level is stored as biomass and transferred to the next level. This principle, often called the “10% rule,” is a consequence of physics.
The energy transformed into heat is no longer available for biological work. This continuous loss dictates that energy must constantly be resupplied to the system, primarily from the sun. Energy movement is a one-way street, preventing it from being reused by the organisms that initially captured it.
Chemical Cycling in Ecosystems
In contrast to energy, the matter that makes up living organisms, such as carbon, nitrogen, and water, is finite on Earth. These essential chemical elements are conserved and move through the environment in biogeochemical cycles. These cycles involve the transfer of matter between the living (biotic) and non-living (abiotic) components of the ecosystem.
Chemicals are stored in environmental compartments called reservoirs, such as the atmosphere, soil, bodies of water, or biomass. For instance, carbon is stored in the atmosphere as carbon dioxide, while nitrogen is fixed into usable forms by certain bacteria in the soil. The movement of these chemicals between reservoirs is driven by physical, geological, and biological processes.
The water cycle, for example, demonstrates a closed-loop movement as liquid water evaporates and returns as precipitation. Decomposers break down dead organic matter, returning elements like carbon and nitrogen to the soil or atmosphere in forms that can be absorbed and reused by producers. Matter is neither created nor destroyed during these transformations.
This conservation of matter ensures that the chemical building blocks necessary for life remain within the ecosystem. The continuous reuse of these elements allows life to persist without needing a constant external input of new atoms.
Why Energy Flows and Chemicals Cycle
The fundamental difference between the movement of energy and chemicals is rooted in the laws of thermodynamics. Energy moves in a linear, one-way path, while matter travels in a closed, circular loop.
The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed. Organisms transform light energy into chemical energy, and then into mechanical energy and heat. The Second Law of Thermodynamics explains why this transformation is unidirectional: every energy transfer increases entropy, converting some energy into unusable heat.
Because energy is lost as heat with each metabolic process, it is no longer available to be recycled by organisms. This continuous dissipation means the ecosystem is an open system requiring a constant influx of solar energy to sustain life. Without this input, the system would quickly run out of usable energy.
Matter is governed by the Law of Conservation of Mass, which dictates that atoms are neither created nor destroyed. Although the chemical form of matter changes, the atoms themselves are retained within the system. The closed nature of biogeochemical cycles means the same atoms can be indefinitely cycled between the biotic and abiotic components of the Earth.
An ecosystem needs a perpetual supply of external energy because energy is continually lost as heat, but it only needs a finite amount of chemical matter because matter is continuously recycled and conserved.